(i) Field of the Invention
This invention relates to the recovery of metal ions from solutions containing other ions. More particularly, it relates to the use of polymeric membranes for proton-driven ion transport for a wide range of ion extraction and separation procedures.
(ii) Description of the Prior Art
Membrane processes have found commercial application in dialysis, desalination and gas separations and hold similar promise for several other separation processes including metal recovery and upgrading. Such membranes for metals recovery would replace conventional solvent extraction or ion exchange units by providing for the continuous removal of the desired metal ion through the membrane. Suitable membranes based on mobile carriers have been widely investigated for metals ranging from alkali metal and alkaline earth cations to copper, chromium and uranium.
One procedure adopted by the prior art is the use of ion retardation resins for desalting solutions. For example, Lee et al., in U.S. Pat. No. 4,235,717, patented Nov. 25, 1980, provided improvements in ion retardation resins e.g. those resins described in an article in Industrial and Engineering Chemistry, Vol. 49, No. 11, November 1957 (pp 1812-1819), titled "Preparation and Use of Snake Cage Polyelectrolytes" by Melvin J. Hatch, John A. Dillon, and Hugh B. Smith. The above patentee provided ion retardation resins said to be particularly useful for desalting caustic solutions which were prepared by employing ion exchange resins consisting essentially of a mixture of a reticular, insoluble, cross-linked styrene/divinylbenzene copolymer with an entrapped non-leachable polymer of acrylic acid, in which the acrylic acid groups on the polyacrylic acid were in substantial excess over the amount needed to react with all the quaternary ammonium groups, which were nuclear substituted on the styrene copolymer chains.
Lee et al., in U.S. Pat. No. 4,376,100, patented Mar. 8, 1983, provided a lithium halide brine purification procedure, i.e. the obtaining of high purity lithium halide solutions, through the use of resin/aluminate composites. This patentee provided an improvement in such procedure by means of a resin/aluminate composite which had been substantially loaded with Li.sup.+ values by being contacted with a contaminated, Li.sup.+ -containing aqueous solution, then being prewashed with a substantially pure, concentrated NaCl brine to remove the contaminants without removing the Li.sup.+ values and then being washed with water to leach out much, but not all, of the Li.sup.+ values. The resin/aluminate composite used comprised a macroporous anion exchange resin having crystalline LiX.2Al(OH).sub.3 dispersed therein, where X was halide.
Another suggested technique for ion separation involved the use of a macrocyclic polyphenol (calixarene) ligand. These cyclic polyphenols comprising a specific type of ring of monomer units were first reported by A. Zinke and E. Ziegler, Chem. Ber., 77, 264-272 (1944). They are somewhat similar in structure to the cyclic polyethers and other macrocyclic ligands which are characterized by their size-related selectivity in binding cations.
Izatt et al., in U.S. Pat. No. 4,477,377 patented Oct. 16, 1984, provided a process of recovering cesium ions from mixtures of ions containing them and other ions, e.g., a solution of nuclear waste materials. The patented procedure involved establishing a separate source phase containing such a mixture of ions, establishing a separate recipient phase, establishing a liquid membrane phase, (contaning a ligand, preferably a selected calixarene) in interfacial contact with the source and recipient phases, maintaining such interfacial contact for a period of time long enough to transport, by the ligand, a substantial portion of the cesium ions from the source phase to the recipient phase, and recovering the cesium ions from the recipient phase.
Blasius et al., in U.S. Pat. No. 4,452,702, patented June 5, 1984, provided a process for the extraction of cesium ions from an aqueous solution with an adduct compound containing a macrocyclical polyether and an inorganic heteropoly acid component. An organic phase in the form of a solution of an adduct compound in a polar organic solvent was first prepared. The adduct compound was the product of a crown ether containing at least one species of particularly specified structural elements, with an inorganic heteropoly acid, which was stable in a strongly acid and oxidizing medium, or a salt of the inorganic heteropoly acid. The aqueous solution containing the cesium ions was brought into contact with the adduct compound to extract the cesium from the aqueous phase into the organic phase. The organic phase charged with cesium ions was then separated from the aqueous solution.
Also, Blasius et al., in U.S. Pat. No. 4,460,474, patented July 17, 1984 provided a process for the extraction of cesium ions from an aqueous solution with an adduct compound in solid form containing a macrocyclical polyether and an inorganic heteropoly acid. The adduct compounds employed comprised an adduct of (a) benzo-15-crown-5 (B-15-C-5); dibenzo-21-crown-7 (DB-21-C-7), or dibenzo-30-crown-10 (DB-30-C-10), with (b) 12-molybdophosphoric acid, 12-tungstophosphoric acid, 12-molybdosilicic acid, 12-tungstosilicic acid, or a sodium, potassium, tellurium or ammonium salt of any of these acids.
Liquid membranes (disclosed by the above prior art) are closely related to solvent extraction processes and potentially could suffer from degradation due to loss of the liquid membrane to the contacted aqueous phases on prolonged use. In contrast, polymeric membranes fabricated with the extractant as part of the membrane structure would not be susceptible to losses of this type. Those fixed site polymer membranes would be similar to conventional ion exchange membranes in which the extractant provides ion binding sites throughout the bulk of the membrane. The present invention aims to provide such an improvement in extraction techniques involving the use of crown ethers by providing such crown ethers in a more commercially viable form, i.e. in the form of a polymeric membrane with fixed sites.
Fixed site polymer membranes for anion transport are known which utilize a variety of polymeric heterocyclic bases as simultaneous proton and anion binding sites (see, for example, M. Yoshikawa et al., J. Membr. Sci. 20, 189-199 (1984) and reference therein). These membranes are capable of sustaining anion-proton cotransport (species move in the same direction) and can be utilized to "pump" anions against their concentration gradient using a pH gradient to drive the process.